Apparatus for the purification of exhaust gas from diesel motors

ABSTRACT

An apparatus for cleaning the exhaust gases of diesel engines, and especially for removal of soot particulates, utilizing series connected filter elements installed in the cross section of a housing traversed by the exhaust gas, wherein at least one filter element carrying a catalyst that lowers the ignition temperature of the soot and assists in soot burn off alternates several times with at least one filter element carrying a catalyst that assists the combustion of gaseous pollutants.

This is a divisional of co-pending application Ser. No. 07/147,603 filedon 1-22-88, now U.S. Pat. No. 4,828,807 which is a continuation of Ser.No. 06/914,416 filed 10-2-86 now abandoned, which is a divisional ofSer. No. 06/703,005 filed 2-19-85 now abandoned.

The present invention relates to an apparatus for cleaning the exhaustgases of diesel engines with respect to oxidizable solid, liquid andgaseous pollutants.

In addition to the pollutants found in the exhaust of Otto (internalcombustion) engines, such as hydrocarbons, oxides of nitrogen and CO,diesel engines because of their mode of operation also emit sootparticulates or extremely fine condensation droplets or a conglomerateof both ("particulates"). These particulates, hereinafter simplyreferred to as "diesel soot" or simply as "soot" are very rich incondensable, polynuclear hydrocarbons, some of which have been found tobe carcinogenic.

In the United States, for example, in order to reduce the particulateemission levels, a limit has been set for the amounts of particulates inthe exhaust gas. Thus, in California, starting with the model year 1986,particulate emission levels shall not exceed the limting value of 0.2g/mile. This cannot be accomplished through measures taken inside theengine alone. Therefore, exhaust gas aftertreatment devices must be usedto provide for a technical solution to this problem.

Proposals have been made in the past to capture diesel soot in filtersand to regenerate the latter by periodically burning the trappedparticulates. As could be determined by thermogravimetric analysis ofparticulates in the filter, the mode of operation of a diesel engineaffects the composition of the diesel soot trapped in filters. The soot,at the relatively high exhaust gas temperatures when the load carried bythe engine is high, contains primarily carbon particulates, while at therelatively low exhaust gas temperatures when the engine load is low,still contains considerable amounts of volatilizable matter in additionto the carbon particulates. Therefore, when the engine load is high,less diesel soot is filtered off from a specified exhaust gas volume,while volatile diesel soot constituents pass through the hot filter ingaseous form.

Under normal operating conditions, the temperatures of diesel exhaustgases are not sufficiently high to burn off the trapped soot. Toaccomplish this, at least 500° to 600° C. is required, depending on thesoot composition which is dependent upon the engine or the load.Therefore, a timely increase of the exhaust gas temperature must beensured for the regeneration of the filter in order to avoid anexcessive accumulation of soot and, thereby, an increase of the exhaustgas back pressure which ultimately leads to choking. This can, forexample, be effected by periodically enriching the air-fuel mixtureformed in the engine and by generating higher exhaust gas temperature.

Another possibility provides for the installation of a burner in theexhaust gas system upstream of the filter which is ignited as necessaryand heats the filter to the temperature required for burning off thesoot.

However, these approaches increase the fuel consumption and hencediminish to a certain degree an important advantage of the dieselengine. German Patent 31 41 713 A 1 and German Patent Application P 3232 729.3 previously proposed the lowering of the ignition temperature ofdiesel engine by providing a soot filter or soot trap with specialcatalysts so that a substantial reduction in the fuel consumption can beachieved during the regeneration phase of the filter. A catalystsuitable for this purpose is, for example, silver vanadate orperrhenate. The catalyst, optionally in combination with a carriermaterial, can be applied to a filter element which, in addition to itsown function, performs the additional function of a structuralreinforcer for the catalyst. Filter systems that are of particularinterest are, for example, a packing consisting of temperature resistantmetal or mineral wool, a filter element disclosed in German Patent 29 44841 A 1 or in German Patent 29 51 316 A 1, or a ceramic monolithtraversed by parallel flow passages in which at any one time a passage,which is open on an end face, is closed on the other end face so thatmacroporously designed passage walls act as filter surfaces.

A characteristic shared by the filter systems equipped with a catalystas described above is that they collect on their filter surfaces, duringthe so called soot phase, the soot, or to be more precise, the exhaustgas portion that can be filtered off after the particular operatingcondition until, with the occurrence of temperarture peaks in theexhaust gas, ignition with the subsequent burn off phase occurs. Thetemperature peaks can be deliberately brought about, e.g. by enrichingthe mixture.

The ignition temperatures are lowered by the catalytic activation withan oxygen content of the exhaust gas of approximately 2-14 percent byvolume, usually by about 50°-80° C., and a complete cleaning of thefilter, heretofore referred to as "complete soot burn off" is achieved.

It has been observed, however, that with the ignition temperaturelowering catalysts complete oxidation cannot be achieved on theonrushing volatile diesel soot particulates or of the diesel sootcollected and ignited at the filter during the soot loading phase andhydrocarbons that may be adsorbed by the soot particulates, usuallymultinuclear aromatics, during burn off can be volatilized or possiblyeven split into partly volatile products, or oxidized incompletely.

As a result, there may be observed not only a considerable increase inthe emission of carbon monoxide and, possibly, even of hydrocarbons thatare volatile at normal outside temperature beyond the emission level ofthe diesel engine which, in and of itself, is deemed favorable, but alsohydrocarbons that are evaporated during the "burn off" and arenonvolatile at normal outside temperature pass through the filter ingaseous form, then condense upon entering the ambient air and therebyincrease to an undesirable degree the particulate emission level. Theburning process can be noticed by a sudden drop of the exhaust gas backpressure upstream of the filter.

According to German Patent Application P 32 32 729.3, attempts have beenmade to solve this problem by coating the filter units describedtherein, which operate according to the dual stage soot loading andburning off cycle on the side of their exhaust gas inlet, with anignition catalyst and, on the side of their exhaust gas outlet, with anoble metal catalyst.

However, the design of these single unit systems which are provided, onthe upstream side of a filter membrane, with the ignition catalyst and,on their downstream side, with the noble metal catalyst, had to beimproved because it is only with difficulty and with an enormous amountof time that the two different layers of catalyst can be deposited onthe front and rear sides of the membrane or membranes of these filters.In addition, they are traversed only once by the exhaust gas to bepurified, so that soot retention as well as afterburning of volatilepollutants is limited and, from the thermal point of view, the boundarysurface between the two types of catalyst is heavily charged.

Therefore, the object of the invention is to provide for each of the twotypes of catalyst an independent filter element as a structuralreinforcer and to connect the different activated separate elements inthe form of a repeating sequence.

Accordingly, the invention relates to an apparatus for purifying theexhaust gases of diesel engines with a filter installed in the crosssection of a housing traversed by the exhaust gas and provided with acatalyst that lowers the ignition temperature of filtered out sootparticulates and assists in its burn off, as well as with a catalystthat assists the combustion of gaseous pollutants. In the housing, thefilters are installed in series for trapping the soot particulates,either directly in contact or spaced a distance from each other. Onefilter element A carries the catalyst that lowers the ignitiontemperature of the soot and assists in its burn off and at least onefilter element B carries the catalyst that assists in the combustion ofgaseous pollutants, and these alternate several times with one another.

In order to protect the catalyst that assists the combustion of gaseouspollutants from inactivation caused by trapped particulates, a preferredembodiment of the invention provides for a series of alternatingseparate filter elements or groups of filter elements of the species Aand B that begins in the direction of the exhaust gas stream with aspecies A and terminates with a species B. This embodiment does not ruleout a reversed arrangement.

As a general rule, filter elements of the species A or B are used in thesame geometric configuration. However, with an appropriate design of thehousing, elements with a different geometric configuration can also beplaced one behind the other. It was found to be beneficial to spacefilter elements A or B with the same geometric configuration a distancefrom one another which distance is, at most, twice the thickness of thefilter element. As a result, as the exhaust gas issues from a givenfilter element, it is vortexed anew into the interspace, therebyadvantageously affecting the conversion of the pollutants.

Due to the cascade type arrangement of several filter elements of thedifferent species, the filtration and conversion of the particulates andthe afterburning of existing or formed gaseous pollutants will not onlybe repeated several times, but the heat generated in the front sectionsof the exhaust gas cleaning device will also be made use of in rearsections. This permits the reduction of the stream resistance of thefilter and, in the case of medium and relatively high operating loads,will produce a practically continuous conversion of the particulates ontheir way through the exhaust gas cleaning apparatus, so that a distinctsoot loading phase can be dispensed with.

Filter elements that may be generally used for purposes of the inventionare: ceramic disks sintered into open porosity; disks from pressedceramic fibers, particularly fibers of Al₂ O₃, SiO₂, aluminium silicate,or ZrO₂ ; disks formed of sintered metal; disks formed of pressed steelwool; packed beds of temperature-resistant ceramic or metallic material.All such materials are known in the art.

Toward this end, a flat disk from compacted wire cloth surrounded, ifdesired, by a metal enclosure by way of holding fixture, can be used asan extremely suitable filter element for a housing having any crosssection, preferably round. The disk can be obtained by axially pressinga hose braided in several layers from an endless wire which is resistantto high temperatures and immune to corrosion. These filter elements aredescribed in detail in German Patent 32 03 237 A 1 and are thus known inthe art.

In another embodiment of the invention which is also of considerablesignificance, the catalyst for filter element A, which is to assist theignition and burn off of the particulates, consists of one or more ofthe following compositions (a) to (e) that have been found to beexceedingly effective for this special case of heterogeneous catalysisof the conversion of a particle shaped material with a gaseous oxidizingagent:

(a) lithium oxide,

(b) vanadium pentoxide,

(c) vanadium pentoxide plus an oxide of one or more of the followingelements:

Li, Na, K, Rb, Cs;

Mg, Ca, Sr, Ba;

B, Al;

Si, Sn;

Sb, Bi;

Cu, Ag;

Zn;

Sc, Y, La, Ce, Pr, Nd, Tb;

Ti, Zr, Hf;

Nb;

Cr, Mo, W;

Mn, Re;

Fe, Co, Ni,

wherein the oxide admixture is preferably 1-30 percent by weightreferred to V₂ O₅,

(d) vanadate of one or more the metals listed under (c) above, and

(e) perrhenate, preferably Li, K, Ag, V.

This type of catalyst can be combined with a temperature resistantcarrier material, the latter being mixed with the catalyst or beingapplied to the filter element, which serves as a base for the catalyst.Suitable carrier materials that can be employed separately or inmixtures are, for example, MgO, Al₂ O₃, particularly γ-Al₂ O₃, CeO₂,SiO₂, SnO₂, TiO₂, ZrO₂, HfO₂, ThO₂, Nb₂ O₅, WO₃, magnesium silicate,aluminum silicate and/or magnesium titanate or combinations thereof.These substances are known in the art and any suitable one may be usedas a carrier.

Of particular significance is the choice of the material for the filterelement A. It has been found that materials with chemical compositions(in percentage by weight), such as the following may be used:

    __________________________________________________________________________    C     Mn P  S  Cr Si V  Co Al Zr  Y                                           __________________________________________________________________________    (a)                                                                             0.09                                                                              0.29                                                                             0.02                                                                             0.02                                                                             12.93                                                                            0.22                                                                             0.03                                                                             0.48                                                                             5.05                                                                             0.15                                                                              0                                           (b)                                                                             0.11                                                                              0.30                                                                             0.02                                                                             0.02                                                                             13.17                                                                            0.22                                                                             0.07                                                                             0.56                                                                             4.92                                                                             0.23                                                                              0                                           (c)                                                                             0.03                                                                              0.31                                                                             0.02                                                                             0.00                                                                             20.0                                                                             0.44                                                                             0.12                                                                             0.48                                                                             4.3                                                                              0.12                                                                              0                                             or also                  5.3                                                (d)                                                                             0.17                                                                              0.28                                                                             0.02                                                                             0.02                                                                             15.39                                                                            0.32                                                                             0.02                                                                             0.34                                                                             5.53                                                                             0.005                                                                             0.45                                        __________________________________________________________________________

Materials (a) and (b) correspond to material 1.4725 (DIN) and material(c) corresponds to material 1.4767 (DIN). Further, iron alloys with ahigh nickel percentage, iron alloys with a nickel coating, as well asiron alloys coated with aluminum or with an aluminum diffusion layer areparticularly suitable. Such substances are known in the art and anysuitable ones may be used for purposes of the invention.

One or more elements of the platinum group, optionally together with oneor more base metals, can be used as the catalyst for the filter elementB, in combination with a temperature resistant carrier material,preferably MgO, Al₂ O₃, particularly γ-Al₂ O₃, CeO₂, SiO₂, SnO₂, TiO₂,ZrO₂, HfO₂, ThO₂, Nb₂ O₅, WO₃, magnesium silicate, aluminum silicateand/or magnesium titanate or combinations thereof, the carrier materialbeing mixed either with the catalyst or applied to the filter elementand serving as a base for the catalyst.

As a material for the filter element B, an aluminum containing chromiumsteel is preferred which can be coated with an adhesive aluminum oxidelayer by tempering in air at temperatures of from 800° to 1300° C. Thisis a material known in the art. Carrier material and catalyticallyactive constituents can then be applied to this anchoring layersimultaneously or one after the other according to methods known in theart.

The invention will now be described with reference to illustrativespecific embodiments in conjunction with the accompanying drawingwherein:

FIG. 1 shows an exhaust gas filter containing 12 filter elements;

FIG. 2 shows the curve of the soot collecting and burning phase in aseries of 12 uncoated filter elements;

FIG. 3 shows the curve of the soot collecting and burning off phase in aseries of 12 filter elements of the species A;

FIG. 4 shows the curve of the soot collecting and burning off phase in aseries of 6 filter elements of the species A followed by 6 filterelements of the species B; and

FIG. 5 shows the curve of the soot collecting and burning off phase in aseries of 12 filter elements of species A and B, in which one element ofthe species B alternate with each other.

In carrying out the specific examples, an exhaust gas filter consistingof a cylindrical sheet metal housing formed of chromium nickel steel wasused in each case. This housing was closed on the front side with acover that tapers conically toward a connecting piece for the exhaustgas inlet or outlet and in each case contained in the cylindrical innerspace, a series of 12 filter elements, one after the other composed ofdisks made from braided wire cloth as disclosed in German Patent 32 03237 A 1 (Knecht Filterwerke GmbH, Stuttgart), wherein these filterelements were in tight fitting engagement with their peripheries againstthe housing wall.

The disk made of braided wire cloth corresponds to the materialdescribed in 1.4767 DIN (German Industrial Standards) has a compositionin percentage by weight of C 0.03, Mn 0.31, P 0.02, S 0.00, Cr 20.0, Si0.44, V 0.12, Co 0.48, Al 4.8 and Zr 0.12 with a diameter of 72 mm, athickness of 3.2 mm, and weight of 10.2 g. It was fabricated by pressingwith a high degree of precision a hose braided in several layers from awire, 75 m long and 0.15 mm thick.

To fabricate filter elements of the species A, the disks of braided wirecloth were first pretempered for 1 hour at 700° C., then rolled at roomtemperature in a powdery mixture consisting of 99 parts by weightvanadium pentoxide (purity 97%) and 5 parts by weight silver vanadate,after which the resultant coating was treated for 30 minutes at 700° C.The result was a sticky, closed catalyst layer, 1-10 μm thick, (catalystmaterial per disk is approximately 2 g).

To fabricate filter elements of the species B, 9 g γ-Al₂ O₃ and 0.07 g,platinum was applied to each disk. The aluminum oxide was applied bydipping the disk into a 30 percent by weight aqueous aluminum oxidedispersion, air-blowing and drying at 200° C.; these operations beingrepeated 3 to 6 times. The catalyst layer was then tempered for 120minutes at 700° C. The platinum was applied by repeatedly impregnatingthe Al₂ O₃ coated disk from braided wire with an aqueous solution of[Pt(NH₃)₄ ] (OH)₂ and drying at 200° C., as well as subsequent 1-hourcalcinating at 500° C. and, finally, a 2-hour reduction treatment in aforming gas (composition: 95 percent by volume N₂ +5 percent by volumeH₂) at 550° C.

For the individual tests, 12 filter elements were placed in uncoatedform, or coated with the species A or B, in the housing depicted in FIG.1.

As shown in FIG. 1, the test converter used in the examples consisted ofa cylindrical housing part 1 formed of material desribed in DIN 1.4571which, on the end of the upstream side, has a flange that projectsperpendicularly to the longitudinal axis of the housing with six equallyspaced holes 5 to receive a screw bolt connection 7.

At the end of the downstream side, the housing is provided with a conethat tapers toward a cylindrical outlet pipe connection for the exhaustgas to which the rear section of the exhaust pipe is attached.

Twelve of the above disk shaped filter elements 2 of the desired speciesare inserted in the housing part 1 in cascade fashion and spaced adistance of 6 mm from one another. Their peripheries are in tightfitting engagement with the housing jacket. The distances between thefilter disks are maintained by inserting intermediate rings 3 made fromDIN 1.4571 material. The spacing serve to improve the mixing of thegases.

On the upstream side of the converter, the housing part 1 is connectedto an upstream head 4 made from the same material. The upstream headconsists of a ring fitted into the inner jacket of the housing withallowance for sliding therein (its length corresponds substantially tothe thickness of the housing flange). This ring is mounted with a conethat tapers toward the inlet pipe connection for the exhaust gas. Theexhaust pipe originating from the engine is plugged into the exhaust gasinlet pipe connection. On the cone of the heat portion, a flange matingwith the flange of the housing 1 is provided with six holes 5' toreceive the so called screw bolt connection 7. A flange of the housingpart 1 is mounted in such a way that its sealing surface lies in theplane going through the ring shoulder on the cone. A sealing disk 6 madefrom asbestos (Montanit) is provided between the two flanges.

By tightening the screw bolts 7, the filter elements 2 are compressedvia the intermediate rings 3. As a result, their peripheries are in evencloser fitting engagement with the inner jacket of the housing, with theresult that a bypass of the exhaust gas is avoided and all the exhaustgas is forced to pass through all the filter elements.

The installed exhaust gas filter is built into the exhaust pipe of adiesel engine provided with a water-eddy-current brake with anelectrical speed and load regulating device:

VW--4-cylinder diesel engine

Power: 40 kW

Stroke volume: 1500 cm³

Maximum speed (without load): 5,550 rpm

No-load speed: 825 rpm.

The exhaust gas analysis includes a determination of HC, CO, NO_(x)upstream and downstream of the exhaust gas filter as well as a carbonmeasurement downstream of the filter by means of an opacimeter(turbidimeter).

The exhaust gas back pressure (loss of pressure) upstream of the filteris measured with a box contact pressure gauge and the exhaust gastemperature upstream of the filter with a Ni-Cr-Ni thermocouple.

For the tests described in the examples, the following measuringprocedure was carried out by means of the testing equipment discussedearlier:

The diesel engine is set at a constant load and rate and operated underthese conditions until a load pressure of 150 mbar is reached at thefilter (collecting phase). The time required for this, as well as otherparameters, is continually logged by a 6-channel printer.

Prior to the start of the actual test cycle, the carbon ignitiontemperature was determined by gradually increasing the load from aloading pressure of 150 mbar until an equilibrium pressure (ignitionpressure P_(i)) is attained. The ignition temperature T_(i) is definedas the exhaust gas at which, under constantly maintained engineoperating conditions, the pressure increases no further; i.e., until thesoot being deposited at the filter or arriving there is immediatelyburned off anew.

After operating 5 minutes under constant conditions, the engine load israised until the regeneration temperature lying above T_(i) is reached.Then, the deposited soot starts to burn and the pressure upstream of thefilter diminishes rapidly until it reaches a limiting valuecorresponding to the degree of regeneration which varies, depending onthe filter type or system. After 20 minutes, the regeneration is deemedtermination and the cycle repeated.

The time from the start of the regeneration until attainment of the endpressure for the evaluation of the catalyst layer(s). This indicates thesoot burn off speed. A mean value resulting from the sum of the singlecycles is used for the evaluation.

In addition to the soot ignition temperature, the trapping of theparticulates is also determined by means of a turbidimeter (opacimeter)downstream of the filter unit (test converter).

EXAMPLE 1

The example describes the course of the particulate collecting andburning off phase in a series of 12 filter elements that are not coatedwith a catalyst.

Twelve filter elements designed in the manner mentioned above, withoutcatalytic coating, were loaded at the testing equipment with dieselexhaust gas in the converter depicted in FIG. 1.

The measured data are apparent from the table shown after the examplesand from FIG. 2.

In the so called collecting phase, which occurs at a constant rate andengine load, the filter elements are loaded with the soot particulatescontained in the exhaust gas, as a result of which the loss of pressure(or the back pressure of the exhaust gas) in the system increases. Whenthe engine load is increased, equilibrium pressure P_(i) and equilibriumtemperature T_(i) gradually set in.

When the engine load is increased further, the temperature of theexhaust gas rises, thereby accelerating the burn off of the sootparticulates. As a result, the curve indicating the loss of pressuredrops relatively steeply and thereafter runs substantially horizontallyuntil the end pressure P_(E) is reached.

The shape of the pressure loss curve depicted in FIG. 2 shows thatignition temperature, ignition pressure and final pressure lierelatively high here. A conversion of volatile hydrocarbons and COcannot be determined. In the filter, the initial particulateconcentration is reduced by 74% through separation, corresponding to aparticulate emission from the filter of 26% of the initial value.

EXAMPLE 2

This example shows the curve of soot collecting and burning off phase ina series of 12 filter elements of the species A.

The filter elements were inserted in the converter and tested on thetesting equipment. The measured data are shown in the table shown afterthe examples.

From this and from FIG. 3, it is apparent that particulate collectingphase, soot ignition and filter regeneration occur substantially as inExample 1, the difference being that in this case the ignition, due tothe catalyst action, already sets in with a smaller loss of pressure andat a lower temperature and, hence, in a shorter time.

The filter was regenerated more fully, which is apparent from theattainment of an end pressure P_(E), which is substantially lowercompared to Example 1.

The hydrocarbon and CO conversion did not change (within the scope ofthe measuring accuracy) compared to Example 1, i.e. the ignitioncatalyst is unable to convert these materials.

The particulate reduction, expressed as a percentage, which is lowerthan in Example 1, is due to the catalyzed soot burn off which leads toa thinner filter coating of correspondingluy poorer filtering ability.

EXAMPLE 3

This example shows the curve of the soot collecting and burning offphase in a series of 6 filter elements of the species A followed by a 6filter elements of the species B.

The filter elements of the species A, followed by the filter elements ofthe species B, were inserted in the converter and tested on the testingequipment. The measured data are apparent from the table shown after theexamples and from FIG. 4.

It was found that the collecting and burning off phases had the samecurves as in Examples 1 and 2, but with the difference that the sootignition sets in with a still greater loss of pressure and at a stilllower exhaust gas temperature.

At the end of the regeneration phase, a still lower end pressure P_(E)was reached. Further, a considerable reduction in the CO and HCconversions could be accomplished with coincident use of filter elementsof the species B. In addition, a slight improvement of the degree offilter action over Example 2 was achieved by combining the two speciesof filter elements.

EXAMPLE 4

This example described the course of the soot collecting and burning offphase when using 12 filter elements of the species A and B, with oneelement of the species A and one element of the species B alternatingwith one another.

The elements were inserted in the converter in the sequence A-B-A-B andthe filter system was tested on the testing equipment. The measured datacan be seen from the table shown after the examples and from FIG. 5.

The shape of the curve depicted in FIG. 5 shows a substantially flatterrise in the loss of pressure throughout the operating time than in thepreceding examples. In this case, the particulate burn off had alreadyoccurred at exhaust gas temperatures when the load carried by the enginewas small, a considerable portion of the onrushing soot particulatesbeing burned off on the filter without substantial collection.

As a result, there is no longer a distinct carbon soot collecting phasehere, which must be deemed a considerable advantage of the alternatingA-B-A-B arrangement.

This examples shows that with an alternating an arrangement of filterelements of the species A and B one can obtain the lowest ignitiontemperatures, the smallest pressure increase and the greatesthydrocarbon and carbon monoxide conversion, as well as a furtherimprovement of the degree of filtering action.

                                      TABLE                                       __________________________________________________________________________    Measured Data Table Pertaining To The Examples                                      P.sub.i  P.sub.E          exhaust gas                                         ignition                                                                           T.sub.i                                                                           end  particulate                                                                         particulate                                                                         conversion                                          pressure                                                                           ignition                                                                          pressure                                                                           reduction                                                                           emmision                                                                            at 400° C.                                                             %                                             Example                                                                             mbar temp. ° C.                                                                 mbar %     %     HC CO NO.sub.X                                __________________________________________________________________________    1 (FIG. 2)                                                                          200  475 110  74    26    *  *  *                                       2 (FIG. 3)                                                                          175  408 70   63    37    *  *  *                                       3 (FIG. 4)                                                                          157  375 65   65    35    72 70 *                                       4 (FIG. 5)                                                                          100  350 60   68    32    87 87 *                                       __________________________________________________________________________     *Values lie within the measuring accuracy, i.e. they are negligibly small

On balance, the examples show that both a series connection of groups offilter elements of the species A and B and an alternating arrangementA-B-A-B, etc. product considerable advantages over filter systems whicheither contain no catalyst or only one ignition catalyst for burning offthe soot particulates. In both cases according to the invention, theloss of pressure caused by the filter unit can be made lower and thesoot can already be burned off at lower engine loads.

For the practical application in a motor vehicle, the present inventionmeans greater operating safety of the soot filtering device, reducedpollutant emission, and lesser fuel consumption of the engine. Thespecific example of the invention described in Example 4 is evensuperior to that of Example 3. However, the latter example is moreeffective than in systems in which parallel connected filter membranescarry on the upstream side the ignition catalyst and on the downstreamside the afterburning catalyst for volatile or volatilized pollutants.

The special compositions of materials developed for the catalyst of thefilter elements A provide a decisive precondition which can also be putto use independently in other circumstances to overcome the sootignition and burn off problems posed by the relatively cool exhaustgases of the diesel engines.

Further variations and modifications of the present invention will beapparent to those skilled in the art from the foregoing and are intendedto be encompassed by the claims appended hereto.

The entire disclosure of German Patent Application P 34 07 172.5 of Feb.28, 1984 is relied on and incorporated herein by reference.

We claim:
 1. An apparatus for cleaning diesel engine exhaust gases containing soot particulates and other pollutants, comprising:exhaust conduit means for transporting diesel engine exhaust; a housing located in said exhaust conduit means; a filter system installed within an interior of said housing, said filter system including a series of filter elements wherein said series of filter elements includes at least one first filter element, said at least one first filter element carrying a first catalyst material that lowers the ignition temperature and assists in the burn off of solid soot particulates filtered out of the exhaust gases travelling in said exhaust conduit means; at least one second filter element, said at least one second filter element carrying a second catalyst material which enhances the combustion of gaseous pollutants travelling in the exhaust conduit means, said first and second catalyst materials having separate and distinct compositions; and said at least one first filter element and said at least one second filter element being arranged in a sequence which ensures that one filter element carrying one of the two distinct catalyst material compositions is between a pair of filter elements carrying the other of the two distinct catalyst material compositions.
 2. The apparatus according to claim 1, wherein the first and second filter elements are of the same geometric configuration and are spaced apart one from another at a distance of up to twice the thickness of a filter element.
 3. The apparatus according to claim 1, wherein said housing is of a circular cross section.
 4. The apparatus according to claim 1, wherein said first and second filter elements are flat disks.
 5. The apparatus according to claim 4 further comprising metal enclosures wherein said flat disks are each surrounded by a metal enclosure and said flat disks are made of a wire cloth that has been compacted.
 6. The apparatus according to claim 1, further comprising metal enclosures wherein said first and second filter elements are flat disks which are each surrounded by a metal enclosure and said disks are made of a wire cloth that has been compacted.
 7. The apparatus according to claim 6, wherein said disks are obtained by axially pressing a hose braided in several layers from an "endless" wire which is resistant to high temperatures and immune to corrosion.
 8. The apparatus according to claim 1, wherein said first catalyst material is selected from the group consisting of:(a) lithium oxide, (b) vanadium pentoxide, (c) vanadium pentoxide plus an oxide of one or more of the elements:Li, Na, Rb, Cs; Mg, Ca, Sr, Ba; B, Al; Si, Sn; Sb, Bi; Cu, Ag; Zn; Sc, Y, La, Ce, Pr, Nd, Tb; Ti, Zr, Hf; Nb; Cr, Mo, W; Mn, Re; Fe, Co, Ni, (d) vanadate of one or more of the metals listed under (c) for the oxide admixture, and (e) perrhenate.
 9. The apparatus according to claim 8, further comprising that the first catalyst material is combined with a temperature resistant carrier material, wherein the carrier material is mixed either with said first catalyst material or applied to said at least one first filter element and serves as a base for said first catalyst material.
 10. The apparatus according to claim 1, wherein one or more elements of the platinum group are used in the catalyst material for said at least one second filter element.
 11. The apparatus as recited in claim 1 further comprising a plurality of said at least one first filter element and a plurality of said at least one second filter element being arranged in an alternating sequence wherein each of said first filter elements is followed in sequential order by one of said second filter elements within said housing.
 12. The apparatus as recited in claim 1, wherein one of said at least one first filter elements is first to come in contact with the exhaust gases travelling in said exhaust conduit means and one of said at least one second filter elements is last to come in contact with the exhaust gas stream. 